The present invention relates to a control system for an infinitely variable belt-drive transmission for a vehicle. U.S. Pat. No. 4,369,675 discloses a control system for an infinitely variable transmission.
The infinitely variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool to control the oil pressure by shifting the spool.
The primary pulley has a Pitot pressure generating device for producing Pitot pressure dependent on engine speed. The Pitot pressure is applied to one axial end of the spool of each valve to urge the spools to shift. On the other hand, the actual transmission ratio is detected by the axial position of the movable conical disc of the primary pulley which represents the running diameter of the belt on the primary pulley. The position of the movable conical disc is transmitted to the other axial end of the spool of the pressure regulator valve by a rod and a link.
The spool of the transmission ratio control valve is shifted in dependency on the opening degree of a throttle valve of an engine and on the engine and on the engine speed (Pitot pressure) to control the amount of oil to be supplied to the servo device of the primary pulley so as to control the transmission ratio. The spool of the oil pressure regulator valve is shifted in dependency on the engine speed and on the transmission ratio to regulate line pressure of the oil. Thus, the transmission ratio can be smoothly and infinitely varied.
In such a transmission, the belt must be tightly engaged with the primary and secondary pulleys so as not to slip on the pulleys. The engagement force of the belt with the pulleys is maintained by the line pressure regulated by the pressure regulator valve. FIG. 5 shows the relationship between the transmission ratio and the line pressure. The line pressure increases with increase of the transmission ratio (low speed range). Further, when the engine speed rises, the line pressure drops as shown by the chain line.
However, the transmission force between the pulleys and the belt is decided not only by the transmission ratio and engine speed, but by engine torque which is represented by the opening degree of the throttle valve.
Accordingly, in the prior art, the line pressure decided only by the transmission ratio and engine speed is unreliable. In order to ensure the engagement of the belt, the line pressure must be set to a high value which corresponds to a pressure to be required at full throttle. Therefore, at part-throttle, the set line pressure is excessively high for transmitting small engine torque, which means a great loss in power for driving an oil pump and excessive of fuel consumption.